temporarily disable transitions for hillshade-illumination-direction

[mollymerp]

temporary workaround for #5934
cc @stevage

light-position wasn't actually transitioning, despite having transition: true in the style spec, so this won't change behavior from the current version.

when I have some spare time I will implement an interpolation function that accounts for light-position and hillshade-illumination-direction for a better solution

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[lbud]

light-position is supposed to be interpolated using its cartesian coordinates

mapbox-gl-js/src/style/light.js

Lines 45 to 51 in da74713

	interpolate(a: LightPosition, b: LightPosition, t: number): LightPosition {
	return {
	x: interpolate.number(a.x, b.x, t),
	y: interpolate.number(a.y, b.y, t),
	z: interpolate.number(a.z, b.z, t),
	};
	}

so that degree thresholds don't cause a problem there. I would rather fix at least light properties than declare them non-transitionable 🙃 #5982